Enthusiasm building around the Democrats already running risks leaving behind candidates who are still flirting with potential runs. But Dumkrieger said Sanders and Biden both have plenty of name recognition and support from previous Iowa caucus bids that likely affords them more time to make a decision.

Many view O'Rourke, 46, who narrowly lost to Republican Sen. Ted Cruz in November's election, as a "young rock star," he said.

But time may be running out on him and any other lesser-known candidates who want to compete in a very crowded field. Polling shows Biden and Sanders enjoy near-universal name recognition in Iowa, but O’Rourke is less known.

Former U.S. Rep. Beto O'Rourke, D-El Paso, appears with Oprah Winfrey for "Oprah's SuperSoul Conversations from Times Square" on Tuesday, Feb. 5, 2019, in New York. The lineup also featured actors Bradley Cooper and Michael B. Jordan and philanthropist Melinda Gates, underscoring the sense of celebrity that surrounds O'Rourke.(Photo: Kathy Willens/Associated Press)

'I think it's time for something new'

On Friday, local leaders shared the stage with Booker at a campaign forum in Waterloo, Iowa.

In the crowd, Waterloo Democrat Marna Creery liked what she heard from the New Jersey senator, but said she'd consider other young candidates like O'Rourke.

O'Rourke, a previous three-term congressman, gained celebrity status as he propelled a long-shot campaign into a close contest against Cruz. Cruz won the Iowa Republican caucuses in 2016 before losing the GOP nomination to Trump.

While O'Rourke has not responded to invitations from local Democratic Party officials, he told Oprah Winfrey on Feb. 5 that he plans to decide whether to run by the end of the month.

"I've got to tell you, and you can tell, I'm so excited at the prospect of being able to play that role," he said.

Still, he has his work to do here: About a third of likely Democratic caucusgoers said they didn’t know enough about him to form a favorable or unfavorable opinion of him, according to a December Des Moines Register/CNN/Mediacom Iowa Poll. Just 4 percent of likely caucusgoers said Biden and Sanders were unknown to them.

A national poll from Morning Consult released this week showed Biden led with 29 percent of respondents choosing him as their preferred candidate for the Democratic nomination. He was trailed by Sanders and California Sen. Kamala Harris.

On Monday, the McClatchy news service reported that interviews with 31 Democratic operatives revealed deep pessimism about a potential Biden run. That story cited the former vice president's age (76), his policy baggage and the field of fresh faces igniting passion in the party.

Those concerns were echoed by others in Waterloo. Democrat Darcy Stoen, 51, of Evansdale, said she liked Biden because of his record as vice president and as a Delaware senator.

“But his age worries me a little bit,” said Stoen, who attended a Booker campaign event there Friday. If Biden runs, she said, she’d like to see him pick a younger candidate like Booker as his running mate, and commit to serving just one term before handing over the reins to a new generation.

He's drawn to both senators because they have targeted the nation's financial industries. He wants to see more regulation of the payday loan industry and see the federal government break up the nation's biggest banks.

Former United States Vice President Joe Biden fires up Democratic voters during a rally for gubernatorial candidate Fred Hubbell and first congressional district candidate Abby Finkenauer during a speech in Cedar Rapids on Tuesday, Oct. 30, 2018.(Photo: Bryon Houlgrave, The Register)

Jason Humphreys, also of Bettendorf, identified Warren as his second choice behind Sanders. He said Sanders "set the standard" in 2016 by advocating for a $15 minimum wage, taking on Wall Street and lobbying against the influence of corporate donors and political action committees

"I would caucus for her and I would fight for her to be president," said Humphreys, 37, "but I’m hopeful for Bernie.”

At a stop in Cedar Rapids, Iowa, Sunday, Robin Jackson Miller, 40, of Hiawatha, Iowa, said she saw similarities between Warren and O'Rourke.

"He’s just very inspiring and seems to be able to talk to people in a way that gets them interested in politics, gets them excited and gets them to care about things," she said.

At the same event, Kelly Kindl said she wanted to vote for a female presidential candidate. But she's also eyeing others, including O'Rourke, Harris and Buttigieg. But she's particularly interested in the former vice president.

"He might have a bigger draw, just because of his history in politics," said Kindl, 40, of Marion, Iowa.

'This thing's going to change a half a dozen times by caucus day'

Most presidential campaigns must start from scratch in building out an organization in Iowa. Biden and Sanders can lean on previous volunteers and organizers.

But staffing may be another question.

"People are excited about Bernie. They would love to work for him, but they need a paycheck," said Evan Burger, an organizer at Iowa Citizens for Community Improvement who worked on the Sanders campaign in 2016. "I don’t think we’ve hit that point yet where people are going to have to start dropping off, but at some point we’re going to run into that."

He said he believes a core group of supporters and staffers will hold out for Sanders.

"It's not realistic and it's not fair to expect every single person who was with the candidate last time to be with them this time," said Pete D'Alessandro, who directed Sanders' 2016 Iowa caucuses campaign.

D'Alessandro said he had no insight on whether the independent Vermont senator would seek the Democratic nomination again.

"It's not too hard for me to wait an extra month," he said. "I have no problem with that. If he's in, I'm in."

Previously, Sanders positioned himself as a more liberal alternative to Clinton, who was widely viewed as the favorite from early on. This time, he would have to compete with Warren and anyone else trying to court the Democrats furthest to the left, D'Alessandro said.

Instead of the two distinct lanes occupied by Clinton and Sanders in 2016, voters and the press may group this year's candidates into four or five distinct groups, he said. The the size of the field may lead some to downplay Iowa and focus on other early states like New Hampshire, South Carolina, Nevada and California.

"Look, this thing's going to change a half a dozen times by caucus day," D'Alessandro said.

The 'biggest field in Iowa caucuses history'

At this point in the race, few Iowa Democrats have committed to one candidate, said Sean Bagniewski, chairman of the Polk County Democrats.

So he thinks it's possible that Sanders, Biden and O'Rourke all could join the race.

"If the mayor of South Bend, Indiana (Buttigieg) thinks he has something to offer, I'm pretty sure a former presidential candidate, a previous vice president and somebody who almost beat Ted Cruz think they can compete," he said.

Bagniewski doesn't expect the field to shrink soon. That's because he expects other potential candidates like Montana Gov. Steve Bullock, Washington Gov. Jay Inslee and California Congressman Eric Swalwell also to enter the race.

"I don't know that it's going to be as big and broad as it is now," Bagniewski said. "But I think it's going to be the biggest field in Iowa caucuses history."

In some ways, the deep roster makes it easier for lesser-known candidates to put up a strong showing in the first-in-the-nation caucuses.

"It's whoever has the biggest minority," Bagniewski said. "You’re not looking to have 51 percent ... You’re just looking to have the biggest small group, which is unusual."